Re: [PATCH v6 2/7] dma: avoid redundant calls for sync operations

[Alexander Lobakin]

From: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
Date: Thu, 9 May 2024 13:44:37 +0200

> From: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Date: Thu, 9 May 2024 13:41:16 +0200
> 
>> Dear All,
>>
>> On 07.05.2024 13:20, Alexander Lobakin wrote:
>>> Quite often, devices do not need dma_sync operations on x86_64 at least.
>>> Indeed, when dev_is_dma_coherent(dev) is true and
>>> dev_use_swiotlb(dev) is false, iommu_dma_sync_single_for_cpu()
>>> and friends do nothing.
>>>
>>> However, indirectly calling them when CONFIG_RETPOLINE=y consumes about
>>> 10% of cycles on a cpu receiving packets from softirq at ~100Gbit rate.
>>> Even if/when CONFIG_RETPOLINE is not set, there is a cost of about 3%.
>>>
>>> Add dev->need_dma_sync boolean and turn it off during the device
>>> initialization (dma_set_mask()) depending on the setup:
>>> dev_is_dma_coherent() for the direct DMA, !(sync_single_for_device ||
>>> sync_single_for_cpu) or the new dma_map_ops flag, %DMA_F_CAN_SKIP_SYNC,
>>> advertised for non-NULL DMA ops.
>>> Then later, if/when swiotlb is used for the first time, the flag
>>> is reset back to on, from swiotlb_tbl_map_single().
>>>
>>> On iavf, the UDP trafficgen with XDP_DROP in skb mode test shows
>>> +3-5% increase for direct DMA.
>>>
>>> Suggested-by: Christoph Hellwig <hch@xxxxxx> # direct DMA shortcut
>>> Co-developed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>>> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>>> ---
>>>   include/linux/device.h      |  4 +++
>>>   include/linux/dma-map-ops.h | 12 ++++++++
>>>   include/linux/dma-mapping.h | 53 +++++++++++++++++++++++++++++++----
>>>   kernel/dma/mapping.c        | 55 +++++++++++++++++++++++++++++--------
>>>   kernel/dma/swiotlb.c        |  6 ++++
>>>   5 files changed, 113 insertions(+), 17 deletions(-)
>>
>>
>> This patch landed in today's linux-next as commit f406c8e4b770 ("dma: 
>> avoid redundant calls for sync operations"). Unfortunately I found that 
>> it breaks some of the ARM 32bit boards by forcing skipping DMA sync 
>> operations on non-coherent systems. This happens because this patch 
>> hooks dma_need_sync=true initialization into set_dma_mask(), but 
>> set_dma_mask() is not called from all device drivers, especially from 
>> those which operates properly with the default 32bit dma mask (like most 
>> of the platform devices created by the OF layer).
>>
>> Frankly speaking I have no idea how this should be fixed. I expect that 
>> there are lots of broken devices after this change, because I don't 
>> remember that calling set_dma_mask() is mandatory for device drivers.
>>
>> After adding dma_set_mask(dev, DMA_BIT_MASK(32)) to the drivers relevant 
>> for my boards the issues are gone, but I'm not sure this is the right 
>> approach...
> 
> If I remember correctly, *all* device drivers which use DMA *must* call
> dma_set_*mask() on probe. That's why we added it there and didn't care.
> Alternatively, if it really breaks a lot of drivers, we can set
> dma_need_sync = true by default before the driver probing. I thought of

Or invert the flag, so that false would mean "it needs sync" and it
would be the default if dma_*mask*() wasn't called.

Chris, what do you think?

> this, but the correct approach would be to call dma_set_*mask() from the
> respective drivers.
> 
>>
>>
>>> ...
>>
>> Best regards

Thanks,
Olek